Binding energy of bilayer graphene and Electronic properties of oligoynes E. Mostaani and N. Drummond Thursday, 31 July 2014
Van der Waals interaction Important contributions to the description of binding energy. caused by nonlocal electron correlation effects. Not described accurately by the current methods such as density functional theory (DFT) with various XC functionals and semi-empirical methods.
Computational detail VMC and DMC. Dirack-Fock pseudopotential. Finite-population errors and time step bias are controlled. Ground state energies for simulation cells including 3 3, 4 4 and 6 6 unit cells. Twist averaging by fitting E(N P, k s ) = Ē(N P ) + b[e LDA (N P, k s ) E LDA ( ))]. Extrapolation to infinite system size by Ē(N P ) = E( ) + cn 5/4 P.
Monolayer atomisation energy Difference between the energy of an isolated, spin-polarized C atom and the energy per atom of monolayer graphene. Method E atom (ev/atom) DFT-LDA 1 8.96 DFT-LDA 2 8.873 DFT-LDA (pres. wk.) 8.883 DFT-PBE 3 7.847 DFT-PBE 1 7.93 DFT-PBE (pres. wk.) 7.873 DMC 4 7.464(10) DMC (pres. wk.) 7.395(3) [1] G. Graziano et al., J. Phys.: Condens. Matter 24, 424216 (2012). [2] M. Hasegawa and K. Nishidate, Phys. Rev. B 70, 205431 (2004). [3] A. Hansson et al., Phys. Rev. B 86, 195416 (2012). [4] H. Shin et al., arxiv:1401.0105v2 (2014).
Binding energy of bilayer graphene Twist-averaged BLG BE (mev/atom) 20 18 16 14 12 10 8 6 4 2 TA AA-stacked TA AB-stacked Non-TA BE (mev/atom) 0 0.02 0.04 0.06-5/4 N P 0 0.01 0.02 0.03 0.04 0.05 0.06-5/4 N P 20 0-20 -40-60 non-ta AA-stacked non-ta AB-stacked
Monolayer atomisation energy BE of BLG (both AA- and AB-stacked) obtained in recent theoretical studies. The layer separation d used in the calculations is given in each case. Stacking Method d (Å) BE (mev/atom) AB DFT-LCAO-OO 1 3.1 3.2 70(5) AB SAPT(DFT) 2 3.43 42.5 AB vdw-df 3 3.6 45.5 AB vdw-df 4 3.35 29.3 AB DFT-D 5 3.32 22 AB DFT-D 3 3.25 50.6 AB DMC (pres. wk.) 3.384 17.7(9) AA vdw-df 3 3.35 10.4 AA DFT-D 3 3.25 31.1 AA DMC (pres. wk.) 3.495 11.5(9) LCAO-OO: Linear combination of atomic orbitals-orbital occupancy. 1 Y.J. Dappe et al., J. Phys.: Condens. Matter 24, 424208 (2012). 2 R. Podeszwa, J. Chem. Phys. 132, 044704 (2010). 3 S.D. Chakarova-Kack et al., Phys. Rev. Lett. 96, 146107 (2006). 4 I.V. Lebedeva et al., Phys. Chem. Chem. Phys. 13, 5687 (2011). 5 T. Gould, S. Lebègue, and J.F. Dobson, J. Phys.: Condens. Matter 25, 445010 (2013).
Binding energy curve
Summary of first part 1 Binding energy of AA- and AB-stacked BLG at their experimental equilibrium separations are 11.5(9) and 17.7(9) mev/atom, respectively. 2 Long-range charge fluctuations are more important in the AB-stacked geometry than the AA-stacked, results in larger finite size errors in AB-stacked.
Electronic properties of oligoynes (Preliminary results)
Carbon Allotropes Binding in carbon sp 3 : Diamond. sp 2 : Graphite, graphene, fullerenes and nanotubes. sp: Carbon chain. Linear carbon chain discovery When? Linear carbon chain discovered in nature as late as in 1968 eventually lead to the discovery of C 60. Where? Recognized in interstellar molecular clouds formed with the explosions of carbon stars, novae and supernovae.
Types of Linear carbon chain Polyyne: Alternating single and triple bonds ( C C ) n. Polycumulene: Double bonds (= C =) n. Oligoynes: stabilization of the polyyne chains by end capping (R-(C C) n -R). Synthesis and characterisation The first and second type may be stable at high temperatures (3000 K) as naturally formed in such environments as shock-compressed graphite, interstellar dust, and meteorites. The third type is artificially produced by different chemical routes Information of carbon chains is mainly based on vibrational spectroscopy of stabilized linear carbon chains. The most convenient method of characterising is absorption spectroscopy.
Properties of carbon chain Specific stiffness 10 9 N.m/kg. More than 2-fold improvement over known materials carbon nanotubes and graphene (4.5 10 8 N.m/kg); and almost 3-fold over diamond (3.5 10 8 N.m/kg). Particular applications, such as molecular wire sensors and nano-sized molecular devices, photophysics and photovoltaic devices. Precursors of soot formation and the intermediates for the synthesis of C 60 and carbon nanotubes.
Why oligoynes? Polyyne ( C C ) n and polycumulene (= C =) n are very fragile and reactive. Exposure to oxygen and water completely destroys these specious. Currently, synthetic of polyyne and polycumulene are based on the high pressure and high temperature which makes it hard to be synthesised. Tendency to undergo chain-chain cross-linking reaction causing the evolution towards an sp 2 phase. Experimental information of carbon chains is mainly based on vibrational spectroscopy of stabilized linear carbon chains.
Controversy in HOMO-LUMO gap of oligoynes 20 E g (ev) 15 10 CAM-B3LYP 1 MP2 2 HF 2 B3LYP 1 LDA 3 GW 3 DMC 5 0 0 0.1 0.2 0.3 0.4 0.5 1/n n is the number of paired carbons ( C C ) in unit cell [1] M. Peach, et. al, J. Phys. Chem. A. 111, 11930 (2007). [2] S. Yang and M. Kertesz, J. Phys. Chem. A. 110, 9771 (2006). [3] A. Al-Backri, et. al, submitted.
Summary QMC can be used to find an accurate band gap of materials whose experimental value are not available. Accurate band gap is an important input for investigating the electronic properties of materials. Still continuing.
Thanks to: Financial support from the UK Engineering and Physical Sciences Research Council (EPSRC). Facilities of Lancaster Universitys High-End Computing facility. Facilities of N8 HPC provided and funded by the N8 consortium and EPSRC